This PR adds automatic backups to Nanobox instances. The database, Redis, and user files are backed up every day at 03:00 (server time) to the data warehouse component which comes with every Nanobox app. Old backups are automatically cleared out, but the number of backups that are left untouched can be configured by setting the `BACKUP_COUNT` environment variable to any integer value greater than 0 (the default is 1).

Also updated `.env.nanobox` to reflect the current `.env.production.sample`.

Needed since merge of #3067.

Because Nanobox doesn't run data components in the same container as the code, there are a few tweaks that need to be made in the configuration to get WebPack to work properly in development mode.

The same differences lead to needing to use `DATABASE_URL` by default in the `.env` file for Rails to work correctly.

Limitations of our `.env` loader for Node.js mean the `.env` file needs to be compiled everywhere in order to work, so we compile it in development, now, too. Also, all the `.env.production` tweaks have been consolidated into a single command.

Finally, since Nanobox actually creates the database when it sets up the database server, using the existence of the database alone to determine whether to migrate or setup is insufficient. So we add a condition to `rake db:migrate:setup` to check whether any migrations have run - if the database doesn't exist yet, `db:setup` will be called; if it does, but no migrations have been run, `db:migrate` and `db:seed` are called instead (the same basic idea as what `db:setup` does, but it skips `db:create`, which will only cause problems with an existing DB); otherwise, only `db:migrate` is called.

None of these changes should affect development, and all are designed not to interfere with existing behaviors in other environments.

* Nanobox Support

- Added support for running Mastodon using Nanobox, both for local development, and for deployment to production
- Dev mode tested and is working properly
- Deployment is undergoing test as of this writing. If it works, this line will be amended to state success; if not, one or more subsequent commits will provide fixes.

* [nanobox] Resolve Deploy Issues

Everything seems to work except routing to the streaming API. Will investigate with the Nanobox staff and make fix commits if needed.

Changes made:
- Also need `NODE_ENV` in production
- Node runs on `:4000`
- Use `envsubst` to commit `.env.production` values, since `dotEnv` packages don't always support referencing other variables
- Can't precompile assets after `transform` hook, but do this locally so it only has to be done once.
- Rails won't create `production.log` on its own, so we do this ourselves.
- Some `start` commands run from `/data/` for some reason, so use absolute paths in command arguments

* [nanobox] Update Ruby version

* [nanobox] Fix db.rake Ruby code style issues

* [nanobox] Minor Fixes

Some minor adjustments to improve functionality:

- Fixed routing to `web.stream` instances
- Adjust `.env.nanobox` to properly generate a default `SMTP_FROM_ADDRESS` via `envsubst`
- Update Nginx configs to properly support the needed HTTP version and headers for proper functionality (the streaming API doesn't work without some of these settings in place)

* [nanobox] Move usage info to docs repo

* [nanobox] Updates for 1.2.x

- Need to leave out `pkg-config` since Nanobox deploys without Ruby's headers - create a gem group to exclude the gem during Nanobox installs, but allow it to remain part of the default set otherwise
- Update cron jobs to cover new/updated Rake tasks
- Update `.env.nanobox` to include latest defaults and additions

* [nanobox] Fix for nokogumbo, added in 1.3.x

Apparently, nokogumbo (pulled in by sanitize, added with `OEmbed Support for PreviewCard` (#2337) - 88725d6) tries to install before nokogiri, despite needing nokogiri available to build properly. Instruct it to use the same settings as nokogiri does when building nokogiri directly, instead of via bundler.

* [nanobox] Set NODE_ENV during asset compile

The switch to WebPack will rely on the local value of the NODE_ENV evar, so set it to production during asset compilation.

* [nanobox] Rebase on master; update Nginx configs

- `pkg-config` Gem no longer causes issues in Nanobox, so revert the Gemfile change which allowed excluding it
- Update Nginx configuration files with latest recommendations from production documentation
- Rebase on master to Get This Merged™

Everything should be golden!